一：界面（用的是之前建的一个界面 省了很多时间<TextView

        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@android:color/transparent"
        android:gravity="center"

        android:textSize="22sp"/>
    <EditText
        android:id="@+id/ev_userName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:digits="1234567890.+-*/%\n()"
        android:hint="请输入用户名" />

    <EditText
        android:id="@+id/ev_password"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="请输入年龄"/>


   <CheckBox
        android:id="@+id/chb"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="记住用户名"/>
        <LinearLayout
            android:orientation="horizontal"
            android:layout_height="0dp"
            android:layout_width="wrap_content"
            android:layout_weight="1">

        <Button
            android:id="@+id/btn_write"
            android:layout_height="wrap_content"
            android:layout_width="wrap_content"
            android:layout_marginTop="10dp"
            android:text="写入"/>



    <Button
        android:id="@+id/btn_login"
        android:layout_marginTop="10dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="读取"
        android:textSize="20sp"/>
二：对其功能的实现
（第一步永远是定义相关组件）

public class MainActivity extends AppCompatActivity {
    private EditText evuserName;
    private EditText evpassword;
    private Button btnLogin;
    private CheckBox chkRemenber;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        evuserName = (EditText) findViewById(R.id.ev_userName);
        evpassword = (EditText) findViewById(R.id.ev_password);
        btnLogin = (Button) findViewById(R.id.btn_login);
        chkRemenber = (CheckBox) findViewById(R.id.chb);
        String userName=readFromFile("data.txt");
        if (!TextUtils.isEmpty(userName))
            btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View arg0) {
                String userName = evuserName.getText().toString();
                String password = evpassword.getText().toString();
                Toast.makeText(MainActivity.this, "用户名:" + userName +
                        "年龄:" + password, Toast.LENGTH_LONG).show();

                if (chkRemenber.isChecked()) {
                  if (saveToFile(userName)) {
                      Toast.makeText(MainActivity.this,"保存成功",Toast.LENGTH_SHORT).show();
                      Intent intent=new Intent(MainActivity.this,MainActivity.class);


                  }
                }

            }
        });
    }
    private String readFromFile(String filename) {
        //1.打开输入流文件
        //2.读取
        try {
            FileInputStream in=openFileInput(filename);
            int length=in.available();
            byte[]data=http://www.mamicode.com/new byte[length];
            in.read(data);
            return new String(data,"utf8");
        } catch (java.io.IOException e) {
            e.printStackTrace();
            return null;
        }

    }

    private boolean saveToFile(String userName) {
        //1.打开文件
            try {

                FileOutputStream out=openFileOutput("data.txt",MODE_PRIVATE);
                //2.写入
                out.write(userName.getBytes());
            //3.关闭文件输出流
                out.close();
                return true;
            } catch (java.io.IOException e) {
                e.printStackTrace();
                return false;
            }}
        private boolean saveToPrefs(String userName){

        SharedPreferences.Editor editor=getSharedPreferences("data.txt",MODE_PRIVATE).edit();
            editor.putString("username",userName);
            editor.putString("password",userName);
            editor.putInt("age",18);
           return true;
    }
    private void readFromPrefs(){
        SharedPreferences preferences=getPreferences("data",MODE_PRIVATE);
        String userName=preferences.getString("userName","");
        if (!TextUtils.isEmpty(userName)){
            evuserName.setText(userName);
        }
        int age=preferences.getInt("age",0);
        evpassword.setText(age);
}

    private SharedPreferences getPreferences(String data, int modePrivate) {
        return null;
    }

prefs